This invention relates to photosensitive materials and their use, and, more particularly, to such materials having enhanced sensitivity to incident radiation.
Photosensitive materials are those whose electrical conductivity or emissivity varies in relation to the wavelength and intensity of radiation incident upon the material. Photosensitive materials are typically semiconductors whose electronic structure is sensitive to the incident radiation. Such photosensitive materials have many applications, one example being a detector for indicating the presence of light, commonly termed a photocell. When a beam of light falls upon such a photocell, the electrical conductivity or electron emission of the photosensitive material in the photocell is altered, and this change serves as the basis for quantitatively measuring the intensity of incident light. Another example is the photodiode, wherein incident radiation changes the conductivity of a p/n semiconductor junction in relation to the wavelength and intensity of the incident radiation, thereby permitting electrical current to flow.
Photosensitive materials are also utilized in vidicons, which are devices used to convert images to electrical signals. The vidicon may be used as the camera in a television. In a typical vidicon, a thin layer of a photosensitive material is deposited upon an electrically conducting, transparent plate. Light from a scene imaged onto the vidicon passes through the plate and strikes the thin layer of photoabsorbing material, thus causing the electrical conductivity or electron emission of the thin layer to vary from place to place because of the variations in the light intensity of the image. To form an image in the photoconductive mode, for example, an electron beam is scanned across the free surface of the photosensitive layer, so that the electrical current reaching the conducting plate at any moment is depending upon the local electrical conductivity of the photosensitive material. This electrical signal is then transmitted to a receiver, and can be reconstructed into an image to be viewed. Thus, the photosensitive material lies at the heart of such a vidicon.
Photosensitive materials are also used in a photoemissive mode, wherein the incident light causes emission of electrons from the surface of the photosensitive material. This mode has particular advantages, including high gain, with a relatively large electron emission per incident photon, and low noise.
A photosensitive material should exhibit high absorption of incident radiation, of wavelengths of interest, with a corresponding large variation in electrical properties due to the absorption. Continuing with the example of the vidicon for imaging incident radiation, it is necessary that the variation in electrical properties of the photosensitive material be large, in order to obtain good contrast in the reconstructed image.
One prior approach to achieving such a high variation in photoconductivity was to make the photosensitive layer thick. However, use of a thick layer reduces the resolution of the vidicon because of electronic and optical diffusion effects. If the photosensitive layer is kept thin, on the other hand, there may be insufficient absorption of radiation to produce the necessary conduction or emission contrast, unless the thin layer absorbs a sufficiently large amount of incident radiation to produce the required electrical variation. There is therefore an ongoing need for photosensitive materials which exhibit increased absorption of incident radiation. Increased absorption allows reduced thickness of the photosensitive layer and a resulting improvement in contrast and resolution for the viewer.
Light and related types of energy exist across a broad spectrum of wavelengths. As an example, visible light ranges from about 100 to about 800 nanometers in wavelength, with violet light having the shortest wavelengths and red light having the longest wavelengths. Ultraviolet radiation has a wavelength below the visible spectrum, while infrared light has a wavelength above the visible spectrum. A number of photosensitive materials have been developed for detecting visible light, and are used in many device applications. Unfortunately, no known materials are strongly photosensitive to all wavelengths of radiation. The photosensitive materials currently used to detect visible radiation are generally relatively insensitive to ultraviolet and infrared radiation.
It is also of interest to detect and image radiation having wavelengths that are not visible to the human eye. These wavelengths include the ultraviolet spectrum at wavelengths below 100 nanometers, and the infrared spectrum at wavelengths of about 800 nanometers to 3000 nanometers for the near-infrared spectrum, and upwardly to about 8000 nanometers for the far-infrared spectrum. Although such radiation is not visible to the human eye, its detection and imaging can be of great importance for such applications as astronomy and photographing the earth's resources from space. The photosensitive materials that are presently used to detect visible light typically have only very weak absorption of ultraviolet and infrared radiation, and therefore do not yield high quality devices such as photodetectors, photodiodes, photocells, and vidicons sensitive to radiation in these ranges. Consequently, many present devices sensitive to non-visible radiation do not have the high output, contrast and resolution found in devices sensitive to visible light.
Accordingly, there exists a need for improved photosensitive materials having high absorption of, and sensitivity to, radiation in the ultraviolet and infrared spectra. Such materials should be compatible with existing device designs, and must be fabricable in thin layers of controlled crystallographic characteristics. They should also be stable to microstructural changes in a vacuum and under the influence of an electron beam. The present invention fulfills this need, and further provides related advantages.